In a normal canning process a liquid or semi-liquid food product optionally containing solids, such as a soup, a cooking sauce, etc. is filled into empty cans through an open end to an appropriate level, leaving a headspace above the product, and the open end of the can is then hermetically sealed with an end closure. The cans and their contents are then sterilised by heating, the heating medium used being usually either steam or hot water at a temperature of typically between 115° C. and 130° C. To achieve this temperature the steam or hot water has to be held at a super-atmospheric pressure, and accordingly the cans and the heating medium are contained in a pressure vessel known as a retort or cooker.
The cans, after filling and closing, are placed in the retort, the retort is closed, and steam or water is introduced. Temperature controllers are usually present on the retort to maintain the heating medium at the desired temperature. While the cans are in the retort, heat from the heating medium is conducted through the container walls and thence passes into the product.
All parts of the food product in the can have to reach and hold sufficient temperature for a sufficient time to achieve so-called “commercial sterility”. A large number of products rely mainly on conduction rather than convection to distribute the heat through the can. Typically cans of 73 mm diameter by 110 mm long have to be held for a process time of 80-90 minutes at 121° C. Once sterilised the can must then be cooled, usually with cold water, to a temperature of about 40° C. before being removed from the retort with the cooling and removal procedure taking around 60 minutes.
The total cycle time for placing the un-sterilised cans in the retort to removing them after sterilisation and cooling can be as long as three hours in conventional retorts. This leads to a maximum throughput of just two or three batches of cans for a given retort each working shift. In other words, the retort undergoes only two or three thermal cycles during each working shift.
The long heating times indicated above can lead to overcooking of the food product especially where it lies adjacent to the container wall. In commercial practice it is already well known to reduce the heating time of food products in a static retort by agitating the can by rotating it whilst in the retort. The rotation of the can has been affected either by rotating the can about its cylindrical axis, or by tumbling the can “end-over-end” about a transverse (diametric) axis through its centre.
The first form of agitation can be generated by rolling cans of circular section about their longitudinal axis and is widely used in “Reel and Spiral” cookers. This method of agitation does not induce very efficient mixing of the can's contents, and results in process times being reduced to about 50% as compared to heating without agitation. “End-over-end” tumbling rotation induces better mixing and process time reduction to about 35%, as compared to heating without agitation, can be expected.
In PCT patent application WO 96/11592 there is described a method in which cylindrical cans are held with their axes horizontal, and are moved with a horizontal axial reciprocating motion during the heating and cooling stages, inducing peak accelerations of 1 g or greater (g being the acceleration due to gravity). This process enables canned foodstuffs to be sterilised in typically tenths of the time taken using conventional heating methods, and thus a retort operated using this process may experience tens of thermal cycles per day rather than just the customary two or three.
Conventional retorts for heating cans are designed for a lifetime of tens of years, on the basis of thermal cycle times of three or more hours. If the methods of WO 96/11592 are used, then thermal cycle times are reduced to typically under one hour or possibly to less than 30 minutes and even down to 15 minutes.
The necessarily rapid changes in temperature to achieve these short cycle times increase the thermal stress loadings on the retort. Such repeated thermal loading of the retort can considerably shorten its service life, principally due to the high thermal shock experienced when cooling water is sprayed onto the cans to cool them. Such spray water can splash or otherwise be redirected onto the interior surface of the retort, causing sudden local cooling which generates internal stresses in the retort wall. These stresses can, after a relatively small number of repetitions, lead to the development of cracks, and even to structural failure of the retort.